1. Field of the Invention
The invention generally relates to electrochemical cells and, more particularly, to lithium battery cells for use in implantable devices.
2. Description of the Related Art
The surgical implantation of electronic devices in humans and animals has become a commonplace occurrence. Such devices are used for a wide range of purposes within the body. The most commonly known of such devices is the cardiac pacemaker. Other well-known implantable devices are employed for stimulating or sensing, or both, with respect to the brain, spinal cord, muscles, bones, nerves, glands, or other body organs or tissues.
Implantable devices are becoming more and more complex and commonly include sophisticated data processing hardware such as microprocessors, or related devices, ROM and RAM memories, LSI (Large Scale Integration) devices as well as other computer hardware. In many cases, information is transmitted to and from the implantable device to external monitoring equipment and such information may include device identification, biological data, parameters of present operation of the device (from previous settings), technical information concerning proper functioning of the device, patient and physician data, up-to-date programming for the device and verification of information transmitted to and from the device.
With more and more data being processed and available within the implantable device, there is a need to transmit more and more data from the implanted device to external devices for analysis, reprogramming of the implantable device, or for other purposes.
As a result of the increase sophistication of implantable devices and particularly due to the increased amount of data required to be transmitted from the device, the need
to provide improved power sources for the implantable devices has increased greatly. -There are, of course, limitations on the design of power cells for use in implantable devices, especially with regard to the size and shape thereof. Furthermore, the power supplies must be highly reliable and be capable of providing an adequate amount of current and voltage for an extended period of time.
One commonly employed type of power supply for use in an implantable device is an electrochemical cell, particularly one employing lithium as an anode material. Typically, within such cells, a lithium metal foil anode is provided in combination with a current collector having a porous carbon cathode material coated thereon. The anode and cathode are mounted to opposing side surfaces of a polymeric separator. The resulting electrode structure is mounted within a housing which is at least partially filled with a liquid electrolyte such as thionyl chloride. To provide maximum efficiency it is preferred that the electrode structure fill a substantial position of the entire internal volume of the housing, as any remaining space does not increase the current or voltage capacity of the Cell but merely represents wasted space. Such is a particular problem for electrochemical cells for use in implantable devices wherein the amount of available space is at a premium.
An example of a spiral wound electrode structure is illustrated in FIG. 1. The spiral electrode structure 10 includes anode and cathode portions 12 and 14 separated by a polymeric separator 16. FIG. 1 also illustrates appropriate positive and negative electrical contacts to the anode and cathode portions. A cylindrical button housing 18 is also illustrated in FIG. 1.
By substantially matching the overall shape of the electrode structure 10 with the interior cylindrical shape of the housing 18, the space within the housing is efficiently used and little or no excess space remains. (It should be noted that, within the illustration of FIG. 1, an electrode structure having only a few windings is illustrated. As a result, the amount of empty space remaining within the cell housing is exaggerated within FIG. 1. In actual cylindrical cells, little or no space remains between the interior side wall of the housing and the electrode structure.)
Another housing arrangement employed for electrochemical cells is a rectangular arrangement. Within rectangular cells, a problem occurs in efficiently packing the electrode cell within the housing. If a circular spiral wound electrode structure is employed, a considerable amount of space remains empty within corners of the rectangular cell. Accordingly, various alternative electrode structures have been developed to allow a more efficient packing of the electrode structure into the rectangular housing.
One such configuration, illustrated in FIG. 2, employs a set of parallel plate anode and cathode electrodes 20 and 22 mounted side-by-side within a rectangular housing 26 with each pair of electrodes separated from each other by a polymeric separator 24. Each individual parallel plate structure preferably has a length equal to the interior dimensions of a rectangular housing 26. The rectangular volume within the housing 26 is completely filled. However, by providing a set of separate anodes and cathodes, separate electrical contacts are required for each set. (The electrical contacts are shown schematically in FIG. 2.) For electrochemical cells for use in implantable devices wherein the overall size of the cell must be quite small, the provision of parallel plates, each with respective electrical connections, is difficult and expensive to achieve and, because of the many electrical contacts, the cell may not be as reliable as desired.
It is generally preferred to provide a single continuous electrode structure, such as employed in the cylindrical cells discussed above. One conventional arrangement for mounting a continuous electrode structure within a rectangular cell is illustrated in FIG. 3. Within the cell of FIG. 3, a single continuous electrode structure 28 having an anode 30 and a cathode 32 is bent into a zig-zag configuration with numerous hairpin blends. The electrode structure 28 is mounted within a rectangular housing 34. Positive and negative electrical contacts (shown schematically within FIG. 3) are provided only at opposing ends of the structure. Although such a configuration requires only a limited number of electrical contacts, the bending of the electrode structure into a zig-zag shape can result in damage to the electrode structure. In particular, tearing may occur in the vicinity of the hairpin bends resulting in loss of active material and in possible internal short circuits through the polymeric separator between the anode and the cathode.
FIG. 4 illustrates another conventional method for fabricating an electrode structure for mounting within a rectangular housing. In the arrangement of FIG. 4, a long thin rectangular anode structure 37 is positioned perpendicular to a long thin rectangular cathode structure 36. Thereafter, the anode and cathode structures are folded, along the arrows shown, to yield a substantially rectangular structure, which is thereafter inserted into a rectangular housing. (A polymeric separator, not shown, is preferably positioned between the anode and cathode structures prior to folding of the structure such that the resulting electrode structure includes a polymeric separator between each adjacent anode and cathode segment.) However, the resulting structure suffers from many of the same disadvantages with the structure of FIG. 3. In particular, the presence of sharp hairpin bends within the electrode structure can result in internal short circuits and in regions of poor electrical contact.
It would be desirable to provide an improved electrode arrangement for use in rectangular electrochemical cells, particularly small cells for use in implantable devices, which avoids the disadvantages of prior art arrangements described above. Aspects of the present invention are drawn to such improved arrangements and to methods for fabricating same.